High-Definition Multimedia Interface (HDMI) is an audio and/or video interface for providing digital content. Previously, several analog interfaces have been described for transmitting analog data from a reproducing device, which will be called a source device in the following, to an output device, which will be called a sink device in the following. For example, SCART interfaces have been used to connect between a video cassette recorder or DVD player and an output display, such as a television set.
HDMI connects digital audio/video source devices, such as set-top boxes, Blu-ray players, personal computers and gaming consoles to sink devices, such as audio equipment and liquid crystal display (LCD) devices, e.g. flat-screen TVs. HDMI supports several video and audio formats so that digital audio and visual content can be transmitted in digital form from a source device to a sink device with basically no deterioration in the transmitted information. HDMI devices comprising source devices and sink devices are manufactured to adhere to the HDMI specifications. Several versions of the HDMI specifications exist, such as 1.0, 1.2, 1.3 and 1.3a, which are published by HDMI Licensing, LLC and may be retrieved from their website www.hdmi.org. All these specifications are hereby incorporated by reference.
Since digital audio/visual content is transmitted from a source device to a sink device, a manipulation of the connection, i.e. at or between the devices, has to be prevented so that digital content may not be copied by an unauthorized third party. Hence, the transmitted content is encrypted by high-bandwidth digital content protection (HDCP), for example. HDCP is an encryption system for protected transmission of digital content developed for HDMI. The HDCP specification “High-Bandwidth Digital Content Protection System”, Revision 1.3 of Dec. 21, 2006 published by the Digital Content Protection LLC is hereby incorporated by reference.
An HDCP device that is permitted access to HDCP content is referred to as an authorized device. An HDCP source device may test if a connected HDCP sink device is an authorized device by successfully completing an authentication protocol defined in the above referenced HDCP specification. In HDCP, different device private keys are used for security in an HDCP device and a unique key selection vector (KSV) is provided as a non-secret unique identifier of the HDCP device associated with the device private keys. It is contemplated that an authorized device in the authentication protocol may become compromised so as to expose the device private keys it possesses for misuse by unauthorized parties. If it is determined that the device private keys have been compromised, the corresponding KSV of that device is placed on a revocation list that the HDCP source device checks during authentication. Other authorized HDCP sink devices are not affected by this revocation because they have different sets of device private keys and different associated KSVs. The HDCP source device is required to manage system renewability messages (SRMs) carrying the revocation list including KSVs.
Currently, revocation lists are used on Blu-ray discs. At least one revocation list is stored on a Blu-ray disc and is provided to the HDCP source device when the disc is read by the HDCP source device.
It is desired that HDMI can also be integrated in mobile devices, such as mobile phones or other small portable devices with preferably a radio interface, such as a netbook. However, HDCP is required for protection of the digital content processed and transmitted.
Revocation lists in a HDCP system may contain a large amount of data, e.g. several hundreds of KSVs corresponding to devices that became compromised or are otherwise unauthorized to output digital content. Thus, revocation lists can occupy a large amount of memory since the revocation lists to be distributed contain individual KSVs, and processing this large amount of data may be complicated, consume valuable resources, and take a long time. This can be particularly problematic due to the limited resources available in mobile devices.
For example, a mobile phone comprises a relatively small memory, which may become occupied by a large amount of data of the revocation lists. Further, when executing the authentication protocol, the HDCP source device receives the KSV of the HDCP sink device and before digital content is provided to the sink device, it has to be checked whether the KSV of the sink device is included in a revocation list.
Therefore, the processor of the mobile phone has to get involved in performing a search whether the KSV of the sink device is present in a stored revocation list or revocation lists, so that processing power has to be committed to process the search and may not be used for other important functions, such as signalling in mobile phones, for example.
Compared to a mobile phone, a Blu-ray disc player may be a more powerful device that is able to readout revocation lists from an external physical medium, the Blu-ray disc, and search revocation lists more quickly. However, in contrast to obtaining revocation lists from physical media, such as Blu-ray discs, mobile devices may need to receive revocation lists wirelessly, possibly with a quite limited data rate so that receiving one or more revocation lists may take a long time.
In summary, the size of a revocation list and/or the number of revocation lists may be large and storing and processing capabilities in mobile devices may be limited by the resources of the mobile device.
However, similar to a Blu-ray disc player, it may be desired to connect a sink device, such as an LCD TV to a mobile device, such as a mobile phone, to output digital content, such as high definition video or other media, in an HDCP system.